When an accident occurs in a utility grid to which wind turbines are interconnected, there is a momentary drop in a voltage in the utility grid. The voltage drop in the utility grid leads to the following events occurring in the wind turbines.
(1) An over speed occurs as a result of a momentary drop in an output power of a power generator.
(2) A high current transiently flows through the power generator.
(3) A high current transiently flows through an electric motor of an auxiliary machine too.
In order to operate the wind turbines in safety in such an event, a control process called feathering is typically employed for shifting a pitch angle of wind turbine blades to set the blades in a feathered position, for example.
Further, when the voltage in the utility grid drops, a stator current and a rotor current of the power generator increase. Thus, in order to suppress an increase in the currents, there is a commonly used method of activating a crowbar circuit and such, short-circuiting a rotor coil of the power generator, and protecting a converter device and such from an overcurrent.
However, in a case in which the voltage in the utility grid is restored when the power generator is in the state as described above, a high current flows and a large torque occurs according to a slip frequency at this time.
FIG. 4 shows a relationship between a torque and a current with respect to a slip frequency of an induction machine. As shown in FIG. 4, it can be seen that both the torque and the current also increase as the slip frequency increases.
As used herein, the slip frequency is a parameter expressed by an equation (1) shown below.Slip frequency=(Synchronous Rotation Speed−Induction Machine Rotation Speed)/Synchronous Rotation Speed  (1)
When the slip frequency is greater than zero, the induction machine functions as an electric motor, and when the slip frequency is smaller than zero, the induction motor functions as a power generator (regenerative operation).